The metaconfig.xml file was added in v6.7.2 of the product and is used to validate most of the configuration file during startup. This means that any obvious invalid configuration will stop the dds services from starting and feedback quickly to the user. Once the services start these will also individually check the configuration (which they do in more detail than the early check) but most problems will be caught by the first checks.
When this metaconfig.xml is not present, then this additional validation doesn’t happen and the services are always started. The problem with this is that, if you have an invalid configuration, it’s possible that one service started (and continues) without complaints while another service quits due to an invalid configuration. That’ll mean that only parts of the dds environment are running and the system will be in a incorrect state.